Field of the Invention
The present invention relates to image processing for detecting the altered regions of an image.
Description of the Related Art
Alin C. Popescu, Hany Farid “Exposing Digital Forgeries by Detecting Duplicated Image Regions” TR2004-515, Dartmouth College, Computer Science (to be referred to as literature 1 hereinafter) has proposed a technique of detecting the alteration of the image captured by a compact digital camera, single-lens reflex camera, or the like. This technique detects altered regions using the feature amount of each partial region of a captured image without embedding additional information such as a digital watermark in the captured image, thereby suppressing image quality degradation or an increase in processing time due to information embedding.
It is often a case that a captured image is altered by copying a partial region of the image to another region of the image (to be referred to as “internal copy” hereinafter). For example, a captured image is altered by copying human images included in the captured image to increase the number of people included in the captured image, thereby making the captured image look better.
When an image is altered by internal copy, the feature amount of a copy-source image region (to be referred to as a “copy-source region” hereinafter) is equal or approximate to that of a copy-destination image region (to be referred to as a “copy-destination region” hereinafter). The technique in literature 1 acquires a feature amount using principal component analysis for each image region (for example, each 4×4 pixel region) of a captured image and detects, as an altered region, a region pair (to be referred to as a “copy pair” hereinafter) equal in feature amount within the same image.
If, however, each altered region formed by internal copy partially includes a flat image region, the technique in literature 1 cannot detect the flat region as an altered region.
A flat image region is an image region like a sea or sky region with almost uniform pixel values (for example, RGB component values) in the region. That is, the feature amount acquired from a flat image region does not become a unique value because of almost uniform pixel values, and hence tends to become similar to the feature amount of another flat image region. This makes it impossible to specify a copy pair which are approximate in feature amount in flat image regions. Such regions are therefore not detected in spite of the fact that the regions are altered regions.